Thermistor



Patented Apr. 18, 1944 THERMISTOR Henry Wolfson and Stanley GardenShepard, London, England, assignors to International Standard ElectricCorporation, New York, N. Y.

No Drawing. Application July 1, 1942, Serial No. 449,352. In GreatBritain August 22, 1941 3 Claims.

The present invention relates to a process of manufacturing heater orthe like resistance coils such as, for example, the heater coils ofindirectly heated resistances or the resistance of a resistancethermometer. More particularly it relates to such resistances which havea pronounced negative or positive temperature coefficient of resistance.Such devices have been, for convenience described as Thermistors.

In one form of construction of indirectly heated thermistors, in whichthe resistance element is in the form of a small bead having attachedconductive lead-wires, a prewound heater coil of small dimensionssurrounds the bead, and is held in the desired position relative to thebead, and is insulated therefrom by means such as an insulating cement.In cases where, for instance, the resistance element is extremely small,say 0.03" dia., exact centering of the bead in the heater coil isdifficult, particularly as the heater coil is many times longer than thebead. In such cases the heat of the coil is not concentrated on theresistance element, and the manufacture of thermistors with reproduciblecharacteristics is difficult.

It is the object of this invention to produce heater coils in which theturns lie closely together, though still separate and insulated one fromthe other, of much smaller dimensions than the heater coils hithertoused and of readily reproducible heating characteristics.

According to the invention the process of manufacturing a resistancecoil of the type specified comprises applying to the coil a material,which after fusion and subsequent solidification has insulatingproperties, and applying heat to fuse the material so that the surfacetension of the fused material pulls the coil turns close togetherleaving a thin layer of the material between successive turns.

In one practical method of carrying the invention into effect a heatercoil which consists of a fine gauge resistance wire such as anickelchromium alloy wire known under the registered trade-mark Nichromeor a similar resistance wire, or tungsten, for example, is dipped intoan aqueous solution, which may be saturated, of sodium biborate or othermetal borate solution or suspension (e. g. lithium, potassium, lead,etc), and is then transferred to a furnace or oven at a temperatureabove the fusion point of the borate but below the melting point of thewire. When the borate fuses, the turns of the coil are pulled togetherby surface tension but are insulated from one another and prevented fromtouching by a thin film of the borate.

The process can also be carried out by using any fusible glass or enamelwhich will exert sufficient surface tension to pull the coil turnstogether, and which will insulate the turns one from the other.

A rigid glazed heater of tube like form is produced in this way, intowhich it is easy to insert the thermistor element with its attached leadwires, and in which accurate centering by means such as an insulatingcement is carried out easily and in a readily reproducible manner.

As an alternative method of carrying the invention into effect, the coilis first dipped into water and then in the finely powdered borate orother fusible material or a mixture of fusible materials, and thenheated in a furnace or oven.

Instead of heating the coil and fusible material in a furnace or oventhe fusion of the material and the shrinkage of the coil may beperformed in a flame.

Whilst typical examples employed in carrying out the process accordingto this invention have been given other combinations will be apparent tothose skilled in the art. It will be observed that the materials must beso chosen that the flexibility of the coil is sufficiently great for thesurface tension of the fused material to pull the turns together.

The advantages of the process of manufacture of heater coils accordingto this invention are: heater coils of the same resistance as heretoforecan be made of much smaller length; the size of the assembly when usedwith resistors, for instance, is reduced; the heater is covered with aninsulating layer and is in the form of a rigid hollow tube,non-uniformities in coil lentgh consequent upon cutting a large lengthinto smaller lengths of equa1 resistance are removed, since all coilshaving the same amount of wire shrink to the same size.

What is claimed is:

1. The herein described method of making preformed heater coil tubes forindirectly heated resistors which consists in applying to a heatercoilhaving the requisite degree of flexibility a material which afterfusion and subsequent solidification has insulating properties andapplying heat to fuse the material whereby the surface tension of thefused material pulls the coil turns closely together to a horned stopposition with a thin layer of the material between the successive turns.

2. The process of manufacturing a preformed layer of insulating materialbetween the sucheater coil tube for example for an indirectly cessiveturns.

heated resistance such as a, thermistor which 3. The process accordingto claim 1 wherein comprises covering a spiral coil of tungsten wire thecoil is composed of a nichrome wire of a fine of fine guage with a metalborate and applying 8 guage and the said material consists of metal heatto fuse the borate so that the surface tenborate.

sion of the material pulls the turns of the coil HElIRY WOLFSON.

together to a homed stop position leaving a thin STANLEY GARDEN SHEPARD.

